You may have to set up a rule allowing access on the port used by the network test. If you don't have direct control over the firewall, you'll have to contact your network administrator. The port number used by the test can be found in the network test dialog. In WIndows 10, you should be prompted automatically to allow PerformanceTest through the firewall at the start of the test. 3rd party firewalls might need to be manually configured.